Conventional silicone release coating typically include a soft elastomeric silicone made up of lightly crosslinked polydimethylsiloxane (PDMS) polymers. The manufacture of the precursors to such systems is based on a re-equilibration of linear and cyclic species. While such systems are usually stripped to remove low molecular weight materials, fugitive PDMS fluids may still exist with these coatings, either due to the non-volatility of these fluids under manufacturing conditions, or through incomplete gel/network formation under high speed cure conditions. Such fluids under the right conditions and concentration will spontaneously spread due to their very low surface tension.
In many pressure sensitive applications the subsequent adhesion of a tape or label after it has been in contact with a release coating is used to test the impact of fugitive fluid transfer to the adhesive. Problems occur when fugitive PDMS fluids transfer to the tape or label as the weak boundary layer formed prevents adequate adhesion developing in the final application. For this reason many manufactures of sensitive electronic devices have not used PDMS based release coatings in their manufacturing process to avoid surface contamination.
Glassy silicone-based hard coating are based on sol-gel chemistry entirely or in combination with colloidal particles and typically contain T or Q units. Hence, no fugitive PDMS fluids are available for contamination. These hard coats typically have moduli in the 1-10 GPa range. The drawback to such systems is that they form hard brittle coatings mainly of utility as abrasion resistant coatings; to which PSA labels and tapes will adhere. These hard coats are not suitable for high speed release of conventional pressure sensitive adhesives found in the label and tape industry.
Conventional release coatings are designed for easy release not printability. Where free-standing electronic devices need to be printed, the surface properties of the release coating put severe demands on the ink formulation. For example, elastomeric polydimethylsiloxane (PDMS) release coatings are highly effective at releasing printed adhesives, coatings, and inks. Their low surface tension makes printing directly on to these substrates difficult without encountering de-wetting; this often manifesting as craters. Using surfactants and additives to solve these problems can create new problems; e.g., intercoat adhesion problems in multilayer construction. It would be highly desirable to have a flexible substrate commensurate with a high speed printing process that allows good print definition of electronic devices and easy release without excessive use of formulation aids. A printer friendly release coating is highly desirable.
There is also the possibility of contamination of the printed device by liquid gel-fractions or fugitive PDMS fluids as residue from the production and cure of the release coating. Printing on low surface energy release coatings is possible if the inks are correctly formulated with surfactant and additives. However, these additives tend to cause problems in multilayer constructions; as described above they tend to give intercoat adhesion problems, and they also tend to be profoamers causing macro-foam and micro-foam. It would be desirable to have a flexible substrate commensurate with high speed printing process that allowed good print definition and easy release without excessive use of formulation additives.
The low surface energy of soft conventional pressure sensitive release coatings cause conventional epoxy and acrylate based inks to dewet rapidly due to their higher surface tension. This dewetting occurs rapidly leading to poor film print quality which is replicated on subsequent printing layers. The combination of rigidity and slightly higher surface energy leads to a larger window of printability, without the aid of surface active agents. Surface active agents capable of promoting wetting on low surface energy substrates such as conventional soft elastomeric release coatings have a high incidence of inter coat adhesion problems; the surface active agents bloom to the air surface and are either cured in if they contain appropriate reactive groups or form a liquid low surface tension layer, which can be smudged or transferred by contact, or prevent subsequent adhesion through the weak boundary layer formed all of which compromise the final device quality or function.